1. Field of the Invention
The present invention relates to a spread spectrum coding method and a spread spectrum coding modulator thereof, and more particularly, to a method and a apparatus using the direct sequence spread spectrum (DSSS) technique.
2. Description of the Related Art
Generally speaking, the spread spectrum modulation technique is a method for modulating a signal that have a transmission bandwidth much larger than the original bandwidth of the signal.
FIG. 1 illustrates a transmitter of a conventional communication system implementing the base band direct sequence spread spectrum modulation. Direct sequence spread spectrum (DSSS) modulation is a spread spectrum modulation technique. In FIG. 1, a data signal b(t) is generated from a data source 102, and a spreading code c(t) is generated by a spreading code generator 104. The spreading code c(t) spreads the data signal b(t) using a spreader 106 to obtain a transmission signal m(t) as shown in the formula below.m(t)=c(t)*b(t)
FIG. 2 illustrates a receiver of a conventional communication system implementing base band direct sequence spread spectrum modulation. A received signal r(t) received by the receiver is composed of the transmission signal m(t) sent from the transmitter and an interference signal i(t) as shown in the formula below.r(t)=m(t)+i(t)
The received signal r(t) can be reconstructed to obtain the original data signal through demodulation. A demodulator mainly comprises a multiplier 202, an integrator 204, and a data wave detector 206. The multiplier 202 uses the same spreading code c(t) as the transmitter to demodulate the received signal r(t). The output of the multiplier 202 then is:z(t)=c(t)*r(t)=c(t)*c(t)*b(t)+c(t)*i(t);wherein c(t)*c(t)=1,therefore,z(t)=b(t)+c(t)*i(t).
Apparently, the data signal b(t) is seen in the output signal of the multiplier 202, but with an extra interfering term c(t)*i(t). A low pass filter (i.e., the integrator) 204 with a pass band corresponding to the data signal b(t) is chosen to filter out the interfering term in that the data signal b(t) is a low frequency signal and c(t)*i(t) is a high frequency signal. Finally, the data signal b(t) can be recovered from the output of the data wave detector 206.
Spreading code c(t) is normally Pseudo random Noise (PN) sequences, and sequences generated using non-linear encoding techniques are usually preferred for acquiring better security. Some examples of spreading codes generated by non-linear encoding techniques are maximal codes, and gold codes. All spreading code sequences obtained from these encoding techniques have an odd number of bits in each spreading code. As a result, the number of 0s and that of 1s in the spreading code are always unequal to each other. That means, each spreading code has a direct current (DC) component and is not a DC-balanced sequence.
The receiver of the communication system using the DSSS modulation of the related art employs the heterodyne radio technique, i.e. the dual conversion technique. A major drawback of this structure is high cost. A lower cost alternative includes a direct conversion radio structure, which uses the DSSS modulation technique to modulate the data signal.
When implementing the direct conversion radio structure, DC offset compensation is required for that the circuit will generate additional direct current (DC). If the transmission signal comprises a DC component, the direct conversion radio structure has to determine precisely which portion of the DC component belongs to the transmission signal, and which portion is generated by the circuit. The DC component generated by the circuit tends to be affected by external factors, for example, time, supply voltage, and temperature, thus DC offset compensation needs to be dynamically performed. As a result, the conventional direct conversion radio structure cannot estimate the exact DC offset and provide accurate DC offset compensation.